<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>


    

    <style>

        body{
            background-color: green;
        }
        div{
            width: 0px;
            height: 0px;
            border-left-style: solid;
            border-left-width: 30px;
            border-left-color: red;

            border-bottom-width: 30px;
            border-bottom-style: solid;
            border-bottom-color: transparent;

            border-right-style: solid;
            border-right-width: 30px;
            border-right-color: transparent;

            border-top-style: solid;
            border-top-width: 30px;
            border-top-color: transparent;


        }

    </style>


</head>
<body>

    <div></div>
    
</body>
</html>